Empirical Heat Transfer Coefficient Research Articles

Transient Thermal Analysis of a Double Duct Solar Roof Chimney Coupled With a Scaled Room

Abstract This research presents a detailed transient experimental and computational study of heat transfer and airflow in a scaled room linked with a double-duct vertical roof solar chimney (SC). The analysis was made in the coupled system, varying the heat flux supplied to the roof SC absorber during the day. Experimental temperature profiles were obtained at six different depths and heights, the empirical heat transfer coefficient was computed for the SC absorber, and the variation of air changes per hour was determined. A good agreement between experimental and numerical temperature profiles and average Nusselt number (Nut) is shown in the latter of average difference of 3.41%. The validated computational model was used to analyze the effect of the transient heating of the SC on temperature fields and flow patterns in the thermal system. Almost symmetrical temperature and y-velocity profiles are formed in the ducts at different hours of the day, with thermal boundary layers of about 4 mm. Correlations of the Nusselt number and air changes per hour (ACH) are provided as a function of the modified Rayleigh number.

DESIGN SCENARIO AND ANALYSIS FOR PRELIMINARY SPECIFICATION OF STEAM GENERATOR IN THE PELUIT-40

The helical steam generator is connected to an HTGR-type nuclear reactor called PeLUIt-40 for steam production. Steam is used to generate electricity and hydrogen. A once-through helical tube bundle was employed because of its ability to endure mechanical stress due to thermal expansion, high resistance to flow-induced vibrations, and better thermal performance compared to a straight tube one. To produce the targeted steam, a design analysis of the once-through helical steam generator needs to be conducted. A quick evaluation method was used to predict the preliminary specifications required for steam production. Simple thermodynamic calculations combined with empirical heat transfer coefficients covering convective and boiling processes at constant pressure were used to carry out the analysis. Two scenarios were conducted to evaluate the design choice based on the previous design of RDE-10.

Influence of Accident-Tolerant Fuel With Steel Cladding for Sustainable Heat Transfer in the Reactor Core of VVER-1200

Abstract Since the events at the Fukushima–Daiichi nuclear power plant, there has been increased interest in developing accident tolerant fuel (ATF) to avoid accidents for light water reactors where Uranium-Silicide-based fuel has an excellent field to minimize the hydrogen hazards. Similarly, steel cladding is at the center of attraction for researchers nowadays. In this research, the feasibility of using Uranium-Silicides (i.e., U3Si, U3Si2, and U3Si5) combined with different types of austenitic steel (i.e., AISI) was investigated to improve the safety performance. A three-dimensional (3D) computational fluid dynamics (CFD)-coded star ccm+ model was used to assess heat transfer performance in the hexagonal fuel assembly of a supercritical water-cooled reactor VVER-1200. Utilizing the computational environment of star ccm+, the test analysis was conducted for a portion of fuel height using the realizable K-Epsilon Two-Layer Wall turbulence model. This analysis showed that the combination of U3Si2 fuel with AISI-348 cladding got superiority over other ATF-AISI fuel-claddings assemblies to use in the reactor core of VVER-1200 because of their lower central fuel temperature value with good mechanical and thermal advantages. This work also derived an empirical heat transfer coefficient equation to guide the relevant future investigations on the thermal analysis of the core.

Temperature rise characteristics for angular-contact ball bearings with oil-air lubrication based on fluid-solid conjugate heat transfer

At present, the thermal analysis of oil-air-lubricated angular-contact ball bearings uses empirical heat transfer coefficients to calculate heat transfer. This approach presents problems such as simulating the actual lubrication flow field and ignoring the internal heat conduction in the bearing ring. This paper proposes a CFD steady-state analysis model of oil-air-lubricated angular-contact ball bearings based on fluid-solid conjugate heat transfer to analyze the flow field and temperature field. A temperature rise test of oil-air-lubricated angular-contact ball bearings was carried out to verify the positive determination of the simulation analysis results. Based on a fluid-solid conjugate heat transfer steady-state analysis model, the effects of lubrication parameters, operating conditions, and rolling element materials on the temperature rise characteristics of oil-air-lubricated angular-contact ball bearings were studied. The research results provide a method for analyzing the temperature rise characteristics of oil-lubricated bearings and provide a basis for the analysis of oil-lubricated bearing life.

Structural Optimization of Fast Scanning Magnets for Proton Therapy Device

A new proton therapy facility is under construction in Huazhong University of Science and Technology. Scanning magnets play an important role in this device. They can form an arbitrary two-dimensional radiation shape conformal with the tumor. In the pencil beam scanning system, the magnetic fields are excited by time-varying currents and the maximum repetition is up to 50 Hz. Therefore, the temperature rises of two magnets caused by eddy currents should be considered. In this paper, the coupling analysis of Multiphysics was carried out by COMSOL electromagnetic thermal module to optimize the magnetic pole structure of the scanning magnet. The tapered air gap of the downstream magnet X was designed to match the deflection envelope of the proton beam after the upstream magnet Y. The pole of the magnet SMX was optimized to meet the requirement of the local and integral fields. The module of natural convection and surface heat radiation were used to replace the empirical heat transfer coefficient in the thermal analysis so that the simulation agrees better with reality. The results show that the accuracy of the heat transfer coefficient should be further validated by comparison with the simulation that is more realistic.

Transient Thermal Behaviors of Ultra-Supercritical Steam Turbine Control Valves During the Cold Start Warm-Up Process: Conjugate Heat Transfer Simulation and Field Data Validation

Abstract Transient thermal behaviors of ultra-supercritical steam turbine control valves during the cold start warm-up process of steam turbine systems were comprehensively studied using conjugate heat transfer (CHT) simulation. The geometrical configurations and boundary conditions used in simulation were identical to the field setup in a thermal power plant. The simulated temperature variations were first validated using measurements by the flush-mounted thermocouples inside the solid valve bodies. The CHT simulation implementing the shear stress transport (SST) turbulence model demonstrated good agreement with the field data, and the overall numerical errors were below 10%; however, the numerical errors of the simulation, which used empirical heat transfer coefficients at the fluid–solid interfaces, reached 40%. The determined temperature differences between the cold valve bodies with the hot steam flow decreased significantly. Specifically, the temperature differences along the inner wall surfaces of the valve bodies decreased to less than 50 °C. Further investigation of the transient heat flux distributions and Nusselt number distributions confirmed that the unsteady flow behaviors, such as the alternating oscillations of the annular wall-attached jet, the central reverse flow and the intermediate shear layer instabilities, enhanced the fluid–solid heat convection process and thus contributed to the warming up of the solid valve bodies.

Analytical Design of Helical Coil Steam Generator for Hot Temperature Gas Reactor

Reaktor Daya Eksperimental (RDE) is a non-commercial Hot Temperature Gas Reactor (HTGR) which is currently being developed by National Nuclear Power Agency of Indonesia (BATAN). Integrated helical coil steam generator attached in the RDE is selected to generate 10 MW steam. In order to achieve the rated power, steam pressure of 62 bar, steam temperature of 798.15K and mass flowrate of 3.54 kg/s are required as the output parameters to be produced by the steam generator. A compact design of a once through counter-flow helical coil steam generator is proposed. Analytical design of the helical coil steam generator is conducted to check the heat transfer area required to achieve the rated power. A simple thermodynamic analysis combined with empirical heat transfer coefficient for convective and boiling process inside the steam generator at constant pressure was performed. It is shown that with inlet water temperature of 418.15K and helium gas inlet of 973.15K at 4.27 kg/s and 34.2 bar as the heating fluid produced output helium gas temperature of 511.42K. Based on comparison with available similar existing HTGR reactor, the calculation overestimate heat transfer surface area by around 20%. Hence, the required area to achieve the rated power for the proposed design is estimated around 60.2-75.3 m2.

Analysis of temperature drops on the wall thickness of a supercritical boiler water-wall tube

The paper presents selected results of numerical computations related to simulations of a supercritical power boiler evaporator operation. A detailed analysis is carried out of temperature drops on the thickness of the water-wall tube and the tube fin. A comparison is also made between the results obtained at the fin division into one and two control volumes. The tube cross-sections analysed along the tube length and including the fins are divided into 24 control volumes, for which 2-D transient heat conduction equations are formulated. The 1-D equations describing the principles of mass, momentum, and energy conservation are solved on the side of the working fluid. A convective condition, described by an empirical heat transfer coefficient, is set on the water-wall tube inner surface. The developed mathematical model is a distributed parameter model. The numerical computations are performed for a boiler operating in a power plant in Poland. The analysis takes account of the non-uniformity of the furnace chamber thermal load along its height.

Effect of heat transfer coefficient of steam turbine rotor on thermal stress field under off-design condition

The precise calculation of temperature and thermal stress field of steam turbine rotor under off-design conditions is of paramount significance for safe and economic operation, in which an accurate calculation of heat transfer (HT) coefficient plays a decisive role. HT coefficient changes dramatically along with working conditions. First, a finite element analysis of rotor model, applied with ordinary rotor materials, has been conducted to investigate the temperature and thermal stress difference along with the change of HT coefficient from 20 W/(m2∙°C) to 20000 W/(m2∙°C). Next, the differentiation between existing empirical formulas has been analyzed from the aspect of physical significance of nondimension parameters. Finally, a verifying case of the cold startup of a 1000MW unit has been proceeded. The result shows that the accuracy of coefficient calculation when steam parameters are low has a greater influence on that of rotor temperature and thermal stress, which means a precise empirical HT coefficient formula, like the Sarkar formula is strongly recommended. When steam parameters are high and HT coefficient is larger than 104 W/(m2∙°C), there will be barely any influence on the calculation of thermal stress. This research plays a constructive role in the calculation and analysis of thermal stress.

Coupled thermal simulation of hydrogen storage tank-Dewar flask system

The thermal effects during hydrogen charge and discharge processes are critical for improving hydrogen storage performance. The heat transfer of the hydrogen storage tank couples with its surrounding Dewar flask in thermal simulation. An additional model for studying the natural convection characteristics in the Dewar flask of the hydrogen storage system is developed and integrated into the system model. The Navier–Stokes equation is used to describe the air flow in the Dewar flask. The thermal dispersion is considered to describe the effective thermal conductivity of the storage tank. In the hydrogen storage tank-Dewar flask system, eight monitoring points in hydrogen storage tank and eight points in Dewar flask are selected to monitor temperature variations. The simulated pressures and temperatures in the hydrogen storage tank-Dewar flask system-agree well with experiments. The simulation results show that the model can well describe the natural convective heat transfer characteristics in the Dewar flask and the hydrogen storage behavior in the porous activated carbon tank, and further prove the feasibility and correctness of the coupled thermal simulation of hydrogen storage tank-Dewar flask system. The heat flow rates from the hydrogen tank and the Dewar flask is obtained by integrations along their boundaries to analyze the heat transfer coefficients. The coupled model is useful for optimizing empirical heat transfer coefficients of hydrogen storage tank.

Heat and mass transfer in a horizontal pipe absorber for a heat transformer

A dynamic model is developed which describes the heat and mass transfer of a horizontal pipe absorber applied to a heat transformer used to purify water. A specific configuration of the pipes is used to evaporate water. This absorber operates with a mixture of LiBr-H 2 O, which is distributed from the top and it descends by gravity through the horizontal pipe manifold, passing from one pipe to another. This dynamic model considers three flow regimes: descendant film over each pipe of the refrigerant fluid, a drop formation regime under each pipe, and a free descendant drop from one pipe to the one below. The mathematical model is based in the mass and energy balances, with empirical heat transfer coefficients. With this model, we can identify the relevance of each flow regime. This model represents a useful tool to study the effect of different operating conditions of the absorption heat transformer.

Analytical and Experimental Study of Combustion and Heat Transfer in Submerged Flame Metal Fiber Burners/Heaters

A theoretical model has been developed to predict the thermal performance of inert, direct-fired, woven-metal fiber-matrix porous radiant burner. The local chemical heat release was modeled by a detailed mechanism, and convection heat transfer between the gas and the solid phases in the burner was described by an empirical heat transfer coefficient. The solid matrix was modeled as a gray medium, and the discrete ordinates method was used to solve the radiative transfer equation to calculate the local radiation source/sink in the energy equation for the solid phase. The fully coupled nature of the calculations without external specification of flame location represents a key advance over past efforts towards modeling of porous radiant burners, because for a given mass flow rate the actual heat loss from the flame determines its position and is not a free parameter. The calculated results for the burner surface temperature, the gas exhaust temperature and the radiation efficiency for a single layer Fecralloy burner were compared with experimental data from this laboratory and reasonable agreement was obtained for a range of operating conditions.

99/00584 Boiler heat transfer modeling using CEMS data with application to fouling analysis

A mathematical boiler heat transfer simulation for coal-fired plants is described. Required model input includes boiler geometry, fuel composition, and limited CEMS data that are typically available. Radiation heat transfer in the furnace is calculated using curve-fits to the Hottel charts. The model employs empirical heat transfer coefficient correlations to evaluate convection heat transfer to various boiler component surfaces. Fouling/slagging can be accounted for by including fouling resistance in the calculation of the overall heat transfer coefficient of each component. Model performance predictions are compared to cases available in the literature. Results from parametric studies are presented.

NUMERICAL SIMULATION OF AN ETHYLENE RE-LIQUEFACTION PLANT

The present paper is concerned with the numerical simulation of an ethylene re-liquefaction plant. The plant is part of a petrochemical complex, set up in the city of Aratu, state of Bahia, Brazil. The cooling effect is obtained through a two-stage cascade cycle, involving two circuits with different working fluids: ammonia in the high pressure side, and ethylene is the low pressure. The main components of the system include: two-stage double acting reciprocating compressors, shell-and-tube heat exchanger, store and flash tanks and control valves. The simulation model was developed by considering the energy and mass conversion laws, the empirical heat transfer coefficient relations, heat exchanger and compressor theories and the thermophysical properties of the working fluids. The result was a set of nonlinear equations solved by a sequential modular technique. Predicated thermodynamic parameters (temperature, pressure and mass flow rate), at different points of the systems, were compared with the available plant operational data, with food agreement being obtained. A parametric anaylsis of the plant was also carried out. © 1997 John Wiley & Sons, Ltd.

Empirical Evaluation of Diving Wet Suit Material Heat Transfer and Thermal Conductivity

Abstract This wet suit material testing program provides a quantitative thermal conductivity and heat transfer analysis, and comparison of various materials used in skin diving and SCUBA diving. Thermal resistance represents the primary subject examined, but due to compressibility of the baseline materials and its effect on heat transfer, this program also examines compression at simulated depth. This article reports the empirical heat transfer coefficients for both thermal conductivity and convection. Due to the limitations of the test apparatus, this analysis must restrict the convection evaluation to an approximately 20-cm-height, free-convection model. As a consequence, this model best simulates the overall heat transfer coefficient of a diver hovering in a horizontal position. This program also includes evaluations of some nonstandard materials in an effort to identify alternative wet suit materials.

Analysis of a Screen Heat Exchanger

Heat transfer in a fluid-to-fluid screen heat exchanger is analyzed from first principles. The screens are treated as an ensemble of pin fins and an empirical heat transfer coefficient accounts for convection heat transfer at the fin surface. Pressure drop and simultaneous axial conduction in the screen matrix and the wall separating the fluid streams are modeled. Expressions are obtained that relate dimensionless length ratios to exchanger effectiveness and pressure drop. The “mesh ratio,” defined as the ratio of fin diameter (d) to spacing (s), prevails throughout the results. The key findings are: (1) the existence of an optimal ratio of fin length (a) to fin diameter that maximizes thermal performance (arising from the competition between the fin-length dependent heat transfer coefficient and fin surface area), (2) increasing a/d greater than optimal increases exchanger length and reduces pressure drop; for a/d less than optimal heat transfer is depressed and pressure drop increased, and (3) the pressure drop is linear with overall Ntu and varies as d−2, (1 + d/s)6, and approximately the square of the mass flow rate per width of exchanger. An exact solution for axial conduction is presented that is valid in the limit of large Ntu and equal fluid capacity rates. Axial conduction is seen to decrease with increasing Ntu and mass flow rates and reduced fin a/d ratio. Predictions from the model are validated by comparing with published effectiveness and pressure-drop data.

Thermal and stress analysis of the first wall with asymmetrically heated non-circular coolant channels

Abstract Temperature and thermal stress distributions for a first wall with non-circular coolant channels are calculated without using an empirical heat transfer coefficient. The energy coolant and heat conduction equation are solved simultaneously. The flow is assumed to be laminar; an approximate extension is made to the turbulent flow. The results indicate that the circumferential non-uniformity of the heat transfer coefficient causes thermal stresses that are higher than those predicted by an empirically determined average heat transfer coefficient. The effects of wall thickness and the shape of the coolant channel are discussed.

Effect of Kapitza resistance on standing surface waves in superfluid helium

We analyze theoretically the resonant frequencies of standing surface waves produced by second sound in4He. In particular, we show that an empirical heat transfer coefficient involved in a recently proposed boundary condition can be related to Kapitza resistance. We also calculate the heat flux within the helium and deduce that the height of the surface waves is strongly frequency dependent.

Boundary conditions for second sound in superfluid 4He

Standing surface waves produced by second sound in superfluid /sup 4/He have been observed optically. A spectrum of 84 frequencies was obtained. The theoretical spectrum claculated by using the conventional boundary condition of zero heat transfer does not agree with the experimental spectrum. Good agreement is obtained by assuming nonzero heat transfer at the boundary. An empirical heat transfer coefficient is defined and calculated. The amplitudes of the surface waves are discussed and related to the heat transfer.

Some Generalizations in Compressible Flow Characteristics

Certain generalizations in compressible flow characteristics are first reviewed including specific solutions to flow with losses in the absence of heat transfer (i.e., Fanno flow), in terms of an empirical loss coefficient, Kf. The analysis is then extended to specific solutions to flow with heat transfer, with and without losses (i.e., isothermal and Rayleigh flows), in terms of an empirical heat transfer coefficient, Kq. All solutions are mapped on generalized plots which, in addition to their utility, exhibit a certain beauty of symmetry and continuity.